INFLUENCE OF THE CARBIDE-PARTICLE SPHEROIDISATION PROCESS ON THE MICROSTRUCTURE AFTER THE QUENCHING AND ANNEALING OF 100CrMnSi6-4 BEARING STEEL VPLIV PROCESA SFEROIDIZACIJE KARBIDNIH DELCEV NA MIKROSTRUKTURO JEKLA 100CrMnSi6-4 ZA LE@AJE PO KALJENJU IN POPU(^ANJU

Bearings are used mostly in the quenched and tempered state, e.g., steel 100CrMnSi6-4 with a microstructure of low-tempered martensite and carbide particles undissolved during the quenching austenitization. The size and density of the particles depend on the spheroidisation annealing which is the standard operation at the beginning of the bearing manufacturing. The particles decrease the grain growth of the austenite and determine the grain size after the quenching. The process of accelerated carbide spheroidisation and refinement (ASR) was developed and is used as a replacement of the conventional spheroidisation soft annealing. The ASR process produces the structure of a ferritic matrix and fine globular carbides. The carbide size is several times smaller in comparison with the conventional soft annealing. This microstructure is better for quenching and tempering and ensures a better bearing performance. The article compares the structures and properties of the quenched and tempered 100CrMnSi6-4 steel pre-treated with the conventional soft annealing and ASR. The smaller ASR particle size allows the use of lower quenching temperatures, ensuring the desired final hardness. Samples in the hardened state were compared, considering the prior-austenite grain size, the carbide-particle size and distribution as well as the hardness.